If you adjust the thumbnail settings in the gallery after images have been uploaded there is no way to refresh/rebuild the thumbnails of images already uploaded to the newly adjusted settings in the admin panel.

This feature only rebuilds 25 thumbnails in a gallery category, and leaves the rest.

For reference, we have 12,000 odd images in some 130 user galleries. The largest such user gallery has 1,300 images in it.

MyBB has a thumbnail rebuild function that was able to successfully rebuild 2,000 attachments to posts. They have a clever automated page refresh method that incrementally converts a batch of thumbnails on each page refresh, which prevents browser/server timeouts.

This is not a critical function for us, it was easy enough to rebuild all the thumbnails using imagemagick command line tools, but I thought it worth flagging that this feature may not work as expected for everyone.